Water reuse: Trends, barriers, and the future

People are starting to realize that water is a precious resource and we have to treat it as such. In a city such as Vancouver, where I live, it can be easy to think that conserving water is an unneeded and even ridiculous concept, especially in March, as we go into our sixth straight month of clouds and rain. Despite this, those of us who design and build buildings need to reconsider “the way we’ve always done things,” and stop thinking of water as an endless resource that we can continue to use wastefully.

When you think about it, basically all water in use today is recycled water. We are not creating water in our municipal water systems, we are just collecting it, treating it and pumping it over long distances, using a lot of energy in the process. Then what do we do with this costly, treated water?  Most of the time, we literally just flush it down the toilet. In fact, the United States Environmental Protection Agency (EPA) estimates that toilet flushing accounts for 41 per cent of domestic water use.

How can we reduce this wasteful use of potable water?  We can bypass all the processing, treatment and pumping done at the water treatment plant, and use recycled water locally. It typically still needs to be treated (to a lesser extent) and pumped, but this can be done much more efficiently and using much less energy on a localized basis.

Aside from toilet and urinal flushing, there are many other possible uses for recycled water, including irrigation and cooling tower water make-up. In buildings with a large process water load, such as vehicle washing or other general washing functions, recycled water is a good way to reduce potable water use.  Basically, any domestic use of water that does not have a risk of human ingestion is a good candidate for recycled water (dogs may drink out of the toilet, but humans usually do not). You can even use the recycled water as a back-up supply for fire protection systems.

Rainwater is currently the most common source for recycled water; however, there are many other sources of water that can be reused. These include condensate from cooling, grey and black water, and process water. As there is a wealth of information available on rainwater recycling, this article will focus on the lesser known and implemented recycling of condensate, grey water and black water.

Almost all modern commercial and institutional buildings are mechanically cooled in some way. During the process of cooling the air down to supply air conditioning, a significant amount of moisture is condensed and is typically just discharged to the storm or sanitary drain. This water is typically quite clean and can be reused without much, if any, treatment. Cooling condensate recovery should especially be considered in hot, humid, climates such as Eastern Canada or Southeastern United States.

Thomas Lawrence, Jason Perry, and Tyler Alsen of the University of Georgia published an article in the ASHRAE Journal in May 2012 that studied the economics of cooling condensate recovery in 47 cities across the U.S. This study evaluated the cooling condensate recovery potential and rates along with the cost of water in these various cities. It found simple payback periods that ranged from one year (Miami, FL) to 1113 years (Spokane, WA). The more condensate that can be recovered, and the higher the cost of water, the shorter the payback time is. Despite this large range of payback periods, the study found that 18 cities had simple payback periods of nine years or less. As the cost of water increases, cooling condensate recovery will become a more viable option.

Grey water is generally defined as the discharge from domestic appliances such as sinks, baths, clothes washers, and other washing functions. Black water is the discharge from toilets and urinals and is typically characterized by the presence of organics and bacteria. Wastewater from kitchens can sometimes be classified as black water due to its organic loading. Depending on the proposed use, grey water can be reused with minimal treatment, while black water must almost always be heavily treated to be reused. The most common uses for recycled grey water include toilet flushing, washing and irrigation. In fact, the nitrogen and phosphorus often contained in grey water can act as nutrients for irrigated plants. In a commercial or industrial building, the definition for grey water can extend to discharge from commercial washing processes, such as vehicle washing.

Many commercial vehicle washing systems now include some recycling of the wash water as a standard option. There will always be some make-up water required to dilute the increasing concentration of detergents and contaminants in the recycled water and to make up for losses due to discharge or evaporation; however, a large percentage of the water used for commercial washing operations can be recycled within the process.

As mentioned above, in order to reuse black water, much more treatment is required. In most cases, black water is basically raw sewage. The organics and other contaminants must be removed before the water can be reused. There are currently two major types of black water treatment available on a commercial scale: packaged active mechanical systems, or passive systems that filter the black water through engineered wetlands. The passive systems require a large amount of available land, whereas the mechanical systems can generally be installed in much smaller areas but require more energy to run. Both types of systems can be quite costly to install.

There are many obstacles that can be encountered when designing and installing a water recycling system. The main challenge is to ensure that the system does not adversely affect the health and well-being of the building occupants. In order to do this, the recycled water system must be treated and clearly identified as non-potable. Many pipe manufacturers now offer special pipe that is designed and specially marked for recycled water use. This pipe is typically a different material and colour than pipe used for domestic water and is clearly marked as “non-potable.” This is the best way to ensure that no one tries to tie into the recycled water system for a potable water use in the future. Another barrier is the cost for recycled water systems. In order to collect and reuse water – no matter what the source – large storage tanks, treatment systems and additional piping need to be installed, sometimes at very high costs. As the cost of city-supplied potable water increases, the economics of recycled water will improve and more systems will be installed.

Another common challenge is the acceptance of recycled water systems by local authorities and plumbing inspectors. Until fairly recently, the use of non-potable water was not clearly defined in many plumbing codes. This led to some confusion and plumbing inspectors creating roadblocks for many of the first systems that were installed. Thankfully, the code and general knowledge and experience with these systems have evolved. There are now standards and guidelines that allow the use of these systems while protecting the health of the public at the same time.

As water becomes more scarce and costly to treat and pump, it will become more important to reuse as much of the water within a building as possible. There are many different sources of water that can be reused, and many different possible uses for this recycled water. <>

Water recycling in practice

The University of Calgary Energy Environment Experiential Learning Project (EEEL) collects and recycles water from a number of different sources, including rainwater, process water and grey water. The process water consists of river water that is used to cool the condensers in the chillers and is collected in the rainwater storage tank. Grey water from washroom sinks is collected and used to dilute the acid waste from laboratories. Typically, this dilution would be accomplished with potable water. Reused river water is also used for cooling tower make-up.